1. Field of the Invention
The present invention relates generally to electron beam apparatus and electron microscopy methods.
2. Description of the Background Art
Optical microscopes, the simplest and most used instruments used to image objects too small for the naked eye to see, uses photons with visible wavelengths for imaging. The specimen is illuminated with a broad light beam, and a magnified image of the specimen can be observed using an eyepiece or camera. The maximum magnification of a light microscope can be more than 1000× with a diffraction-limited resolution limit of a few hundred nanometers. Improved spatial resolution in a optical microscope can be achieved when shorter wavelengths of light, such as the ultraviolet, are utilized for imaging.
An electron microscope is a type of microscope that uses electrons to illuminate the specimen and create a magnified image of it. The microscope has a greater resolving power than a light microscope, because it uses electrons that have wavelengths few orders of magnitude shorter than visible light, and can achieve magnifications exceeding 1,000,000×.
Scanning electron beam microscopes, the most widely used electron microscopes, image the sample surface by scanning it with a tightly focused high-energy beam of electrons in a raster scan pattern, pixel by pixel. In a typical SEM, an electron beam is emitted in a vacuum chamber from an electron gun equipped with a thermionic (tungsten, lanthanum hexaboride), thermally assisted (Schottky, zirconium oxide) or cold field emission cathode. The electron beam, which typically has an energy ranging from a few hundred eV to few tens keV, is collimated by one or more condenser lenses and then focused by the final objective lens to a spot about 1 nm to 100 nm in diameter. The beam is deflected by pairs of magnetic scanning coils or electrostatic deflector plates, sweeping in a raster fashion over a rectangular area of the specimen surface. When the primary electron beam strikes the sample, the electrons deposit energy in a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than few nm to few μm into the surface, depending on the electron's landing energy and the composition of the specimen. Primary electrons can generate elastically scattered electrons, secondary electrons due to inelastic scattering, characteristic Auger electrons and the emission of electromagnetic radiation. Each of the generated signals can be detected by specialized detectors, amplified and displayed on a CRT display or captured digitally, pixel by pixel on a computer.
Low energy emission microscopes (LEEM) are projection (as opposed to scanning) electron microscopes, and thus resemble a conventional light microscope. In a LEEM, the electron gun forms a broad electron beam that is accelerated to typically 10 to 30 keV and passed through a magnetic prism separator, an energy-dispersive device that separates the illumination and projection optics and bends the beam into the axis of the objective lens containing the specimen. A parallel flood beam then uniformly illuminates the specimen that is electrically biased at approximately the same potential as the cathode of the electron gun, so that illuminating electrons are decelerated in the objective lens, striking the specimen at energies typically between 0 to about 1000 eV. In the opposite direction, i.e. upward from the specimen, the objective lens simultaneously forms a magnified image of the specimen. As the electrons reenter the prism separator, they get bent into the projection optics. The projection zoom optics forms an electron image on the scintillating screen that is then viewed by a CCD camera and further processed on a computer.
LEEM is a powerful parallel imaging technique that provides information about the topmost atomic layer, and is thus ideally suited for the characterization of surface properties. However, when a conventional LEEM instrument is used to image insulating specimens, the low landing energy exacerbates charging effects resulting in significantly reduced image quality. The imbalance between the arriving and leaving flux of electrons causes the surface to charge up, resulting in increased blur and distortions. In many cases, the built-up surface charge can rapidly discharge in an arc, resulting in specimen damage. On a homogeneous insulator surface, the charging can be suppressed by operating at a landing energy resulting in a net electron yield of 1. However, this approach restricts the landing energy and typically does not work when different insulating materials are present on the surface. Effective means for controlling local surface charging are therefore desirable if LEEM instruments are to be used for imaging of insulating samples.
The dual illumination beam approach is a practical solution to this problem. In a dual-beam LEEM, two electron beams with different landing energies are used to mitigate the charging effect. When an insulating specimen is illuminated with a low-energy electron beam with landing energy near 0 eV, a fraction of electrons is mirrored and the remainder is absorbed, charging the surface negatively. When a high-energy electron beam (˜100 eV or more) is used for illumination, secondary electrons are emitted and the electron yield can exceed 1, charging the surface positively. However, when two beams with opposite charging characteristics, i.e. a low-energy mirror electron beam and a high-energy electron beam are superimposed on the specimen, charging effects can be neutralized. The challenge is to devise an electron optical system that can deliver overlapping illumination of the low-energy mirror and high-energy electron beams at preferably normal incidence on the specimen, i.e. a system that combines two parallel electron beams with different energies and beam currents at the specimen surface.